Wireless technologies such as cellular, Personal Communication Systems (PCS), Multiuser Multipath Distribution Systems (MMDS), Wireless Local Loop (WLL) etc. have inherently limited ranges dependent upon transmitting power, antennas gain, path loss, noise, data rate, etc. In locations where wireless connectivity is required, yet the reception signal power is low, the use of repeaters is prevalent.
Repeaters are bi-directional amplifier devices that receive a low power signal using the receiving (Rx) antenna (first antenna), amplify it and then re-transmit the signal using a higher power signal via a transmitting (Tx) antenna (second antenna). Many repeaters also filter out the noise from the received signal in order to get a clean transmission signal.
Every repeater employs at least two antennas: a receiving antenna (also known as “donor” antenna) which communicates with the networks base stations and a transmitting antenna (also known as “service” antenna) which communicates with the remote stations and subscribers. Having these two antennas in proximity to each other may create a feedback loop when the signal that is transmitted from the “service” is received by the “donor” antenna, filtered, amplified and then transmitted again by the “service” antenna. This creates a positive feedback loop, which causes interferences and disruptions to the regular transmissions, in some cases this can jam nearby cellular base station receivers (BTS's).
In order to avoid a feedback loop, it is required to have a margin of about 10-15 dB between the isolation between the antennas and the repeater gain. For example, if the isolation between the antennas is 65 dB, the maximum gain of the repeater needs to be less than 50-55 dB.
Isolation between the antennas is usually achieved by physically separating the antennas. The “donor” antenna is sometimes installed outside of the area where the repeater is located, which may lead to problems due to wall penetrations and zoning issues. Even when no external antennas are used, separating the antennas may require physical separation of at least 2 meters.
Another option used to overcome the feedback loops is by using echo cancellation algorithms which estimate the feedback signal received by the “donor” antenna and subtracting it from the signal transmitted by the “service” antenna.
FIG. 1 of the prior art is a schematic block diagram of a first typical prior are repeater system (PARS) 10 which uses echo cancellation to overcome the feedback loop.
Prior to operation (and possibly periodically during operation) the prior art repeater system (PARS) 10 moves to a training mode in which a switch 116 connects a training signal generator 106 to a transmit (Tx) filter 108 instead of connecting an adder 114 to the Tx filter 108 (which is the normal, operational connection) and the training signal generator 106 transmits a training signal through the switch 116, the transmit (Tx) filter 108 and a digital-to-analog converter (DAC) 110 to a second antenna 216.
The training signal and a cancellation filter control 112 are also supplied to a cancellation filter 124 for use in the cancellation of the feedback signal.
Prior art repeater systems (PARS) 10 use white noise as the training signal. The PARS 10 sends out a White Noise signal, which essentially blocks out any communications in the vicinity of the PARS 10 until the estimation algorithm (usually located within the cancellation filter 124) converges.
The second antenna 216 transmits the training signal via the feedback channel 118 (which is normally the air). Some of the transmitted signals' power is received by a first antenna 214 and is supplied to an analog-to-digital converter (ADC) 102 and to a receive (Rx) filter 104. The filtered signal is then goes to an adder 114 which subtracts a cancellation signal coming from the cancellation filter 124. The result of the subtraction is fed back to the cancellation filter 124 to determine if adjustments to the cancellation signal are required.
In order to achieve the correct cancellation signal, the energy of the signal coming from the adder 114 must be minimized, this means that the estimated cancellation filter 124 is similar to the feedback path 118, when in full training mode since there is no real data signal being transmitted, the only signal received by the first antenna 214 should be the signal transmitted by the second antenna 216 which is the training signal. In cases where the incoming signals are sufficient, the incoming signals can be used for training.
Due to changes in the feedback channel 118, such as small movements of the antennas, changes in reflection elements such as moving trees or cars, it is required to perform the procedure described above periodically according to the physical requirements to maintain stability. Adaptive algorithms can be used to correct the transfer function in a continuous manner using the incoming signals.
A typical repeater includes transmissions in both directions, but for simplicity, only one direction was described herein and below.
Normally, the filters in the repeater systems (such as the Rx filter 104 and Tx filter 108) are designed to comply with telecommunication standards in a way which will minimize their impact on adjacent frequency bands and also have minimal impact on signal integrity, (minimal propagation delay and amplitude ripple). Such filters are typically band pass filters and the compliance requirements cause the filters to be very close to ideal filters. As most of the filters are implemented as Finite Impulse Response (FIR) filters, the resultant filters become very long and complex filters. As the cancellation filter 124 estimates the signal path from the splitting point 126, it needs to estimate these complex filters (Rx filter 104 and Tx filter 108) therefore, requiring it to be a very long and complex filter itself.
FIG. 2 of the prior art is an example of a first typical band-pass filter magnitude response graph, whose pass-band is 10 MHz, and which attenuates 40 dB in 200 KHz, and 60 dB in 300 KHz.
Such a filter may require 400 taps at a processing frequency of 50 MHz.
FIG. 3 of the prior art is an example of a second typical band-pass filter magnitude response graph, whose transition band is widened to 1 MHz, and may be implemented with 100 taps.
It is possible to see that this band-pass filter is further away from an ideal band-pass filter than the filter described in FIG. 2.
FIG. 4 of the prior art is an example of a second prior are repeater system (PARS) 10 which uses echo cancellation to overcome the feedback loop.
In this second example, the Tx filter 108 is placed after the digital-to-analog converter (DAC) 110 and is therefore an analog filter; typically, a Surface Acoustic Wave (SAW) filter.
The main advantage of SAW filters is that they are easy and relatively inexpensive to manufacture. Yet, since SAW filters are hard-wired by nature, it is impossible to change their profiles once they are built.
From the perspective of the cancellation filter 124, there is little difference (if any) between the two repeater systems depicted in FIG. 1 and FIG. 4, as the complexity of the Tx filter 108 remains high and requires that the cancellation filter 124 to be long and complex.
All of the prior art repeater systems operate at the native radio-frequency (RF) bands which are at the 0.7-5 GHz range. These high frequencies generally yield highly complex designs which can be difficult to implement.
None of the prior art repeater systems include a solution for a simple, inexpensive and programmable cancellation filter which can operate at relatively low frequencies.
There is therefore a need for a repeater system, which comprises a combination of all of the above characteristics and functions.